1. Field of the Invention
This disclosure is generally related to power inverters, and particularly to inverters that invert power from photovoltaic sources.
2. Description of the Related Art
A photovoltaic cell is one of the cleanest and environment-friendly non-conventional energy sources. A photovoltaic cell directly converts solar energy into electrical energy. The electrical energy produced by the photovoltaic cell can be extracted over time and used in the form of electric power. This electric power can be used to drive electric devices. Typically the power is extracted by use of DC-DC up/down converter circuitry and/or DC/AC inverter circuitry.
The popularity of photovoltaic energy generation is rapidly increasing worldwide. One reason for such popularity is that the energy produced by photovoltaic energy generation is essentially pollution free, unlike conventional energy sources such as fossil fuel burning thermal power plants, nuclear reactors, and hydroelectric plants which all raise environmental issues. However, there are difficulties encountered with photovoltaic energy generation which are not present in conventional energy generation systems. These issues include the peculiar IV droop characteristics of photovoltaic cells, the cost, and the relatively low energy density (efficiency) of photovoltaic cells.
FIG. 1A is a graph of P-V curves for various P-V characteristics illustrating a unique difficulty associated with photovoltaic energy generation. Specifically, FIG. 1A shows the Power-Voltage (Power extracted versus Voltage) characteristics of different types of photovoltaic cells. The peculiar IV droop characteristics of photovoltaic cell arrays cause the output power to change nonlinearly with the current drawn from photovoltaic cells. It is clear from the curves that all types of photovoltaic arrays show nonlinear Power-Voltage curves. Furthermore, beyond the fact that the Power-Voltage curves are different for different types of photovoltaic arrays, the Power-Voltage curve changes for different radiation levels and temperatures of operation for any given photovoltaic array.
FIG. 2A is a graph of P-V curves for a single panel of photovoltaic cells operating at different ambient temperatures of 25, 50, and 75 degrees Centigrade. As noted above, the Power-Voltage curves for the same photovoltaic array are different at different temperatures. The near optimal point at which to operate photovoltaic arrays is at or near the region of the Power-Voltage curves where the Power is greatest. This point is denominated as the Maximum Power Point (MPP). It is difficult to track this MPP because the MPP differs across different types of arrays (such as shown in FIG. 1), differs for the same array based on different temperatures (such as was shown in FIG. 2A), and also differs for the same array based on the amount of radiation to which the array is exposed as shown for the curves of the amorphous photovoltaic cells in FIGS. 1A and 1B (e.g., a sunny versus a cloudy day). Consequently, a need exists for a method and system which will track and/or adjust to the MPP in response to variations of the operating conditions and also to the differences in photovoltaic arrays.
Photovoltaic cells are still relatively expensive and it's energy conversion efficiency is still relatively low so a wide area is required to generate sizable power. Hence it is important to operate the photovoltaic cells around the maximum power point to enhance the utilization of photovoltaic cells.
Various methods have been proposed, such as curve fitting techniques, incremental conductance estimation techniques, power matching schemes, Power Voltage slope detection, and using switching frequency modulation.
The power matching technique works well only when the characteristics of the solar panel can be matched with the load characteristics. The power matching technique only approximates the location of the specific radiation level and load conditions. The curve-fitting techniques require prior examination of the solar panel characteristics, so that an explicit mathematical function for describing the output characteristics is formulated. Although this technique attempts to track the MPP without explicitly computing the voltage-current product for the panel power, the curve-fitting technique cannot predict many characteristics including other complex factors, such as aging, temperature, and a possible breakdown of individual cells.
Power-Voltage slope detection techniques need very clean information regarding voltage and power change. This requires a very slow filter in the power calculation and voltage measurements which may detrimentally limit the response time. Thus, a rapid change in isolation level may cause problems in systems employing such techniques.
The switching frequency modulation technique requires a converter between the load and the solar cell, which is switched with a variable frequency to match the resistance with the panel resistance. Consequently, such an approach is not cost effective.